Swift Combine - Observe change of object's property publisher inside an array of objects

Question

I have a following structure:

class FileUploadCellViewModel {
    @Published var isUploaded: Bool = false
}

class FileUploadScreenViewModel: ObservableObject {
    @Published var viewModels: [FileUploadCellViewModel] = []
    @Published var isSendButtonEnabled: Bool = false
    private var cancellables: Set<AnyCancellable> = []

    init() {
        let publisher = $viewModels
            // ?? return a single publisher with `true` if all `isUploaded` are `true`
        }
        let isDateCorrectPublisher = ...

        publisher
            .combineLatest(isDateCorrectPublisher)
            .sink {
                isSendButtonEnabled = $0 && $1
            }
            .store(in: &cancellables)
    }
}

let screen = FileUploadScreenViewModel()
let viewModel1 = FileUploadCellViewModel()
let viewModel2 = FileUploadCellViewModel()

screen.viewModels.append(viewModel1) // expected: isSendButtonEnabled: false
screen.viewModels.append(viewModel2) // expected: isSendButtonEnabled: false

viewModel1.isUploaded = false // expected: isSendButtonEnabled: false
viewModel2.isUploaded = true // expected: isSendButtonEnabled: false
viewModel1.isUploaded = true // expected: isSendButtonEnabled: true

How do I observe each insertion into the viewModels array and after an element is inserted, observe its isUploaded property?

I have found a convenience tool to collect an array of publishers, maybe it would be helpful here.

i think `FileUploadScreenViewModel ` must confirm to `ObservableObject` to publish changes — Wahid Tariq, Nov 01 '22 at 08:55
Yes, I forgot to copy this when I prepared this snippet. It is an `ObservableObject` in the original implementation. — Kacper Cz, Nov 01 '22 at 13:00

score 0 · Accepted Answer · answered Nov 01 '22 at 10:02

You are on the right track with needing a CombineLatestCollection for combining all isUploaded values of all cell view models.

My answer uses the combineLatest operator from the article you've linked.

Once you've combined all publishers, you simply need to call allSatisfy on the [Bool] to see if all cell view models finished uploading.

Now the only part missing is updating the subscription whenever the viewModels array changes - you can do this by observing the $viewModel publisher and passing in the updated array to a method, which combines the isUploaded properties of the updated view models.

class FileUploadScreenViewModel {
  @Published var viewModels: [FileUploadCellViewModel] = []
  @Published var isSendButtonEnabled: Bool = false

  private var subscriptions = Set<AnyCancellable>()

  init() {
    // Whenever the viewModels array changes, set up the subscription on each element of the array
    $viewModels.sink { [weak self] viewModels in
      self?.bindIsSendButtonEnabled(viewModels: viewModels)
    }.store(in: &subscriptions)
  }

  // Combine all isUploaded values from each element of viewModels and update isSendButtonEnabled accordingly
  private func bindIsSendButtonEnabled(viewModels: [FileUploadCellViewModel]) {
    let areUploaded = viewModels
      .map(\.$isUploaded)
      .combineLatest
      .map { areUploaded in
        areUploaded.allSatisfy { isUploaded in
          isUploaded == true
        }
      }

    let isDateCorrectPublisher = Just(true)

    areUploaded
      .combineLatest(isDateCorrectPublisher)
      .map { $0 && $1 }
      .assign(to: &$isSendButtonEnabled)
  }
}

Thank you! I just needed that final mix and match of these methods :D — Kacper Cz, Nov 01 '22 at 13:14
Every time the array has a new element added or removed from it the previous reference is still attached to it — Sajede Nouri, Jul 05 '23 at 08:28

Swift Combine - Observe change of object's property publisher inside an array of objects

1 Answers1